1. Technical Field
The present invention relates to a physical quantity sensor sensing a physical quantity and outputting an electrical signal whose level corresponding to a level of the physical quantity, and in particular, to a physical quantity sensor equipped with a single physical quantity transducer to output a signal indicative of a plurality of ranges of physical quantity.
2. Related Art
At present, life-saving automotive airbag systems have become very popular as means for occupant crash protection. One type of such automotive airbag systems is a side-impact airbag system to protect occupants against a collision impact acting on the right or left side of a vehicle.
The side-impact airbag system is equipped with a left side-impact airbag and a right side-impact airbag. It is required that both of the left and right side-impact airbags operate against shocks acting on a vehicle on the left and right sides thereof, respectively, without introducing errors and without fail.
In order to achieve the above steady operation, acceleration sensors are mounted on a vehicle to detect a collision shock as a physical quantity. A plurality of acceleration ranges are previously given to determine a result to be detected by those acceleration sensors. By way of example, two acceleration ranges are used; one is given to determine a larger-amplitude signal that corresponds to a direct shock occurring when the vehicle causes a collision and the other is given to determine a relatively smaller-amplitude signal that corresponds to a secondary shock caused secondarily by the direct shock but reduced in amplitude, due to being more or less absorbed by the vehicle body during the transmission of the direct shock therethrough.
For example, when a collision occurs on the right side of a vehicle, a vehicle-right-located acceleration sensor (right main sensor) detects a direct shock described above and other sensors other than the right sensor, for example, a vehicle-central-located or vehicle-left-located acceleration sensor (right safing sensor) detects a secondary shock. This detection leads to a determination that there occurred a heavier collision on the right side of the vehicle. Therefore, in this case, only the right side-impact airbag is operated forcibly. Hence it is surely prevented that the right side-impact airbag, which is located on the vehicle's shock-applied side, does not operate and/or the left side-impact airbag, which is arranged to be opposed to the right one, may operate erroneously.
In the conventional side-impact airbag system, not only a pair of main sensors to be disposed on the right and left sides of a vehicle but also one or more safing sensor should be arranged at each of a plurality of locations of the vehicle. Such plural locations exist in the lateral direction (corresponding to a detection-axis direction of each acceleration sensor) at each of frontal, central and back parts of the vehicle's room. This arrangement increases the number of acceleration sensors and the amount of harness to an ECU (Electrical Control Unit), and results in the problem that mounting the side-impact airbag system to a vehicle is largely limited concerning mounting locations.
One solution to the above problem is detecting different magnitudes of acceleration in the same detection-axis direction. This detection can be realized if each acceleration sensor detects accurately an acceleration falling in each of a plurality of ranges. It is thus understood that such a way for the detection will allow a side-impact airbag system to reduce the number of acceleration sensors as a whole.
The idea that uses one acceleration sensor for a plurality of different applications has been taught by Japanese patent laid-open publication No. HEI10 (1998)-282136. This publication discloses a system in which a single acceleration sensor detects an acceleration level and outputs a plurality of types of acceleration signals, of which detection ranges differ from each other and of which response frequency range characteristics also differ from each other. The plural different applications consist of for instance an ABS (Anti Lock Brake System) and an on-vehicle airbag system.
However, the system disclosed by the above publication has a problem of not being able to detect, in particular, a lower-level acceleration with precision. This problem derives from the fact that, in the above system, an output from the signal acceleration transducer is subjected to amplification at two amplifiers in turn and a drift signal component included in the transducer output signal is also amplified as well.
This kind of problem is also true of other physical quantity sensors other than the acceleration sensor, which are for example sensors for an angular velocity and a pressure relating to vehicle's driving behaviors.